1. Field of the Invention
This invention generally relates to electronic visual display devices and, more particularly, to a color-tunable plasmonic display device that creates a “black” color by splitting an incident optical spectrum.
2. Description of the Related Art
Reflective display or color-tunable device technology is attractive primarily because it consumes substantially less power than liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. A typical LCD used in a laptop or cellular phone requires internal (backlight) illumination to render a color image. In most operating conditions the internal illumination that is required by these displays is in constant competition with the ambient light of the surrounding environment (e.g., sunlight or indoor overhead lighting). Thus, the available light energy provided by these surroundings is wasted, and in fact, the operation of these displays requires additional power to overcome this ambient light. In contrast, reflective display technology makes good use of the ambient light and consumes substantially less power.
A number of different reflective display technologies have been developed, such as electrophoretic, electrowetting, electrochromic displays, and interference-based MEMS display. These display technologies all have disadvantages or challenges that must be overcome to obtain greater commercial success. Many existing technologies rely upon phenomena that are intrinsically slow. For example, electrophoretic or electrochemical techniques typically require particles to drift or diffuse through liquids over distances that create a slow response. Some other technologies require high power to operate at video rates. For example, many reflective displays must switch a large volume of material or chromophores from one state to another to produce an adequate change in the optical properties of a pixel. At video switching rates, currents on the order of hundreds of mA/cm2 are necessary if a unit charge must be delivered to each dye molecule to affect the change. Therefore, display techniques that rely on reactions to switch dye molecules demand unacceptably high currents for displaying video. The same holds true for electrochromic displays.
A second challenge for reflective displays is the achievement of high quality color. In particular, most reflective display technologies can only produce binary color (color/black) from one material set. Because of this, at least three sub-pixels using different material sets must be used when employing a side-by-side sub-pixel architecture with fixed colors. This limits the maximum reflected light for some colors to about ⅓, so that the pixels of this type cannot produce saturated colors with a good contrast.
Some reflective displays face reliability problem over a long lifetime. In particular, to sustain video rate operation for a few years requires at least billions of reversible changes in optical properties. Achieving the desired number of cycles is particularly difficult. in reflective displays using techniques based on chemical reactions, techniques that involve mixing and separation of particles, or MEMS technology that involves repeated mechanic wear or electric stress.
FIG. 1 is a partial cross-sectional view of nanoplasmonic display in which the color tuning is accomplished by electrical modulation of the refractive index of an electro-optical material such as a liquid crystal (pending art). Details of the device 100 can be found in the pending application entitled, COLOR-TUNABLE PLASMONIC DEVICE WITH A PARTIALLY MODULATED REFRACTIVE INDEX, invented by Tang et al., Ser. No. 12/614,368. Because of the limited refractive index (n) change of dielectric 106 materials such as liquid crystal, the color tuning range of a device using just this tuning modulation means is very limited. Thus, the device of FIG. 1 uses an additional color tuning mechanism, as described below.
FIG. 2 is a graph simulating the relationship between resonant wavelength change and refractive index for a liquid crystal material surrounding an Ag nanoparticle with a diameter of 80 nanometers. For example, the highest birefringence liquid crystal commercially available only has a Δn of 0.3, which provides a tuning range of only 80 nm, based on the simulation result in FIG. 2. Research labs have reported liquid crystals with a Δn as high as 0.79, but the performance of such materials is not guaranteed. Besides, these materials may not have the appropriate response time or threshold voltage required for the nanoplasmonic display application.
As noted above, the birefringence effect of liquid crystals creates an effective refractive index change that is smaller than the nominal value Δn. This results in a smaller wavelength tuning range than predicted, if an isotropic dielectric medium is assumed. Also, the reflected color has a strong angle dependence if the liquid crystal molecules are uniformly aligned with the electric field passing through the liquid crystal medium. In order to achieve a commercially successful product, both challenges need to be overcome.
In the above-mentioned displays, light wavelength tuning is primarily based on the modulation of the refractive index of liquid crystal in response to an applied electrical field. However, because of the limited index tuning range of liquid crystals, it is particularly challenging to achieve the “black” state, for which the plasmonic resonance needs to be tuned out of the visible wavelength range.
It would be advantageous if a black color state could be achieved for a reflective display without significantly tuning the index of refraction of a medium.